United have been monitoring Costa for over a year but interest from other European clubs has forced the English champions to step up their efforts.

The move for Costa follows the signing of other Brazilian youngsters Anderson, the Da Silva twins, Rafael and Fábio, and Rodrigo Possebon.

Reports suggest Costa has a £21m release clause in his contract, although United hope to bring that figure down.

Costa's agent Cesar Bottega said yesterday: "There have been informal conversations and I believe United would match those values"